Chestnut gall wasp

Host tree

Identification

The numerous galls are easily observable on chestnut leaves and petioles. They are green-red in spring and summer and often remain as brown galls in winter.

Attacked shoots and leaves show reduced growth.

Heavily infested trees show a decline in crown density.

Adult wasps are very small and eggs in buds are hardly visible.

Damage

Heavy attacks lead to severe yield reduction both in chestnut orchards and in forests.

Severely attacked trees show signs of decline. It is not clear whether repeated defoliation may kill trees directly but attacked trees are more prone to chestnut blight.

Since D. kuriphilus is newly invasive in Europe, a proper assessment of its long term impact on the economy, society and environment is needed.

Biology

There is one generation per year.

Eggs overwinter in buds.

Galls containing several larvae develop in spring on young shoots and leaves, reducing their growth.

Adults fly from mid-May to July depending on the climate and lay eggs in new buds.

Risk factors

All chestnut trees in Europe are at high risk, although some cultivars appear more resistant.

Distribution

Dryocosmus kuriphilus originates from China and was first found in Italy in 2002, from where it spread to other European countries: France, Switzerland, Slovenia, Hungary, Croatia, Czech Republic, Austria, the Netherlands, Germany, etc. Also invasive in the USA.

Pest management

Monitoring

Dryocosmus kuriphilus is a quarantine organism in Europe. The movement of chestnut plants in and from infested regions is regulated.

Preventive measurements

New infestations are usually only seen when galls are formed on the leaves, i.e. too late for eradication.

Curative control

Chemical control is difficult because larvae feed internally.

A biological control programme is presently being carried out in Italy and in France though the importation of Torymus sinensis,a specific parasitoid originating from China.

Not all chestnut varieties are similarly susceptible to the wasp. The development of resistant varieties is presently being considered.

Climate change

It is too early to assess the effect of climate change on this insect because it has not yet reached its potential distribution in Europe and its trophic relationships are still evolving.